Chemokines constitute a family of small cytokines that are, inter alia, produced in inflammation and regulate leukocyte recruitment, activation and proliferation (Baggiolini, M. et al., Adv. Immunol. 55: 97-179 (1994); Springer, T. A., Annu. Rev. Physiol. 57: 827-872 (1995); and Schall, T. J. and K. B. Bacon, Curr. Opin. Immunol. 6: 865-873 (1994)). Chemokines are capable of selectively inducing chemotaxis of the formed elements of the blood (other than red blood cells), including leukocytes such as neutrophils, monocytes, macrophages, eosinophils, basophils, mast cells, and lymphocytes, including T cells and B cells. In addition to stimulating chemotaxis, other changes can be selectively induced by chemokines in responsive cells, including changes in cell shape, transient rises in the concentration of intracellular free calcium ions (Ca2+), granule exocytosis, integrin upregulation, formation of bioactive lipids (e.g., leukotrienes), expression of cytokines, and respiratory burst, associated with leukocyte activation, growth and proliferation. Thus, the chemokines are early triggers of the inflammatory response, causing inflammatory mediator release, chemotaxis and extravasation to sites of infection or inflammation.
Two subfamilies of chemokines, designated as CXC and CC chemokines, are distinguished by the arrangement of the first two of four conserved cysteine residues, which are either separated by one amino acid (as in CXC chemokines SDF-1, IL-8, IP-10, MIG, PF4, ENA-78, GCP-2, GROα, GROβ, GROγ, NAP-2, NAP-4, I-TAC) or are adjacent residues (as in CC chemokines MIP-1α, MIP-1β, RANTES, MCP-1, MCP-2, MCP-3, I-309). Most CXC chemokines attract neutrophil leukocytes. For example, the CXC chemokines interleukin 8 (IL-8), platelet factor 4 (PF4), and neutrophil-activating peptide 2 (NAP-2) are potent chemoattractants and activators of neutrophils. The CXC chemokines designated MIG (monokine induced by gamma interferon) and IP-10 (interferon-γ inducible 10 kDa protein) are particularly active in inducing chemotaxis of activated peripheral blood lymphocytes. CC chemokines are generally less selective and can attract a variety of leukocyte cell types, including monocytes, eosinophils, basophils, T lymphocytes, granulocytes and natural killer cells. CC chemokines such as human monocyte chemotactic proteins 1-3 (MCP-1, MCP-2 and MCP-3), RANTES (Regulated on Activation, Normal T Expressed and Secreted), and the macrophage inflammatory proteins 1α and 1β (MIP-1α and MIP-1β) have been characterized as chemoattractants and activators of monocytes or lymphocytes, but do not appear to be chemoattractants for neutrophils.
CC and CXC chemokines act through receptors that belong to a superfamily of seven transmembrane spanning G protein-coupled receptors (Murphy, P. M., Pharmacol Rev. 52:145-176 (2000)). This family of G-protein coupled receptors comprises a large group of integral membrane proteins, containing seven transmembrane-spanning regions. The receptors may be coupled to G proteins, which are heterotrimeric regulatory proteins capable of binding GTP and mediating signal transduction from coupled receptors, for example, by the production of intracellular mediators. Additionally chemokine receptors may act independently of G protein coupling. For instance the Duffy receptor expressed predominantly on red blood cells is a promiscuous chemokine binding receptor which is believed to act as a chemokine, removing chemokines from the circulatory environment.
Generally speaking, chemokine and chemokine receptor interactions tend to be promiscuous in that one chemokine can bind many chemokine receptors and conversely a single chemokine receptor can interact with several chemokines. There are a few exceptions to this rule; one such exception has been the interaction between SDF-1 and CXCR4 (Bleul et al., J Exp Med, 184(3): 1101-9 (1996); Oberlin et al., Nature, 382(6594): 833-5 (1996)). Originally identified as a pre-B cell growth-stimulating factor (Nagasawa et al., Proc Natl Acad Sci USA, 91(6): 2305-9 (1994)), SDF-1 has been the only reported human ligand for CXCR4. The SDF-1 gene encodes two proteins, designated SDF-1α and SDF-1β, by alternative splicing. These two proteins are identical except for the four amino acid residues that are present in the N-terminus of SDF-1β and absent from SDF-1α.
There are many aspects of chemokine receptor signaling and selectivity for ligands that were not previously understood. For example, there are a number of orphan receptors for which no function has been previously determined. RDC1, for example, though earlier thought to be a receptor for vasoactive intestinal peptide (VIP), until recently has been considered to be an orphan receptor because its endogenous ligand has not been identified. See, e.g., Cook et al., FEBS Letts. 300(2):149-152 (1992).
Recently, RDC1, renamed as CCX-CKR2, was determined to bind to both chemokines SDF-1 and I-TAC. See, e.g., PCT/US04/34807 and U.S. patent application Ser. Nos. 10/698,541, 10/912,638 and 11/050,345 each of which are incorporated by reference in their entirety.